The present invention relates to a distance measuring apparatus for use with cameras, video cameras, and the like for measuring the distance to an object.
One technique for the measurement of the distance to an object, the passive method, uses two light receiving sensors having the same optical system. These detect the contrast of an object in order to determine the distance in accordance with a shift in position caused by the parallax of the object as projected on each light receiving sensor. The passive method utilizes ambient light. Thus, it is possible to perform accurate distance measurement if the object is bright enough for the light receiving sensor to detect the object. However, the accuracy decreases when the object does not provide sufficient contrast or is dark.
Another technique for the measurement of the distance to an object, the active method, uses light emitted from a light projecting element towards the object, which is then reflected by the object and detected by means of light receiving elements. The position of the spectral center of the light reflected from the object is used to determine the distance by the triangulation method. With the active method, while distance measurement is possible when the object is dark, has no contrast, or has repeated patterns which reflect light, it is difficult to provide highly accurate range finding for an object located at a relatively large distance away due to a decrease in the quantity of light reflected from the object.
As mentioned above, the passive and active methods have advantages and disadvantages. Therefore, a distance measuring apparatus having both the passive and active methods is desirable in order to obtain cameras having highly accurate distance measuring for various photo taking conditions.
With the active method, since reflected light, derived from a source of light, is detected for performing distance measurement, infrared light is used as the source of light. In this way, the light receiving elements can distinguish that light from ambient light. Thus, the active method allows for distinguishing the reflected light from ambient light or visible light. Consequently, the active method must necessarily use light receiving elements sensitive to infrared light. Thus, optical sensors formed in a line have normally been used as position sensing devices (PSDs).
With the passive method, since the contrast of an object is used for performing distance measurement, light receiving elements are used which are capable of outputting a light and dark contrast as a current value or voltage value, for example charge coupled devices (CCDs).
For this reason, it is necessary to have PSD and CCD light receiving elements, respectively, to facilitate the use of both active and passive methods. Moreover, it is also necessary to have additional optical systems associated with each method, which produces a more complex and larger constitution for the distance measuring apparatus. It is therefore difficult to provide a small and light-weight camera with such a distance measuring apparatus.
It is possible to envisage distance measuring based on both active and passive methods and which uses one of PSD or CCD sensors. However, the passive method will hardly function with PSDs since it is difficult to detect the contrast of the object while, due to the low sensitivity to infrared light of CCDs, it is difficult for CCDs to detect the reflected infrared light radiated from a source of light.
Thus, with the prior art, it has been found that in order to produce a distance measuring apparatus utilizing both methods and their consequent advantages, it has been necessary to constitute individual apparatus. For this reason, the constitution of the distance measuring apparatus has not been significantly simplified.
There is therefore a problem that the distance measuring apparatus can not be miniaturized, can not be reduced in weight, and can not provide any cost savings. Such apparatus can consequently not be applied to compact cameras. In this respect, since such cameras are typically equipped with automatic focusing (AF) devices, the mechanism of which occupies a large proportion of the volume and weight, incorporating a distance measuring apparatus with both methods into this type of camera is not beneficial in terms of improving miniaturization and weight reduction.